The present invention relates to a method and apparatus for treating water by ion exchange.
The present invention is particularly directed to such a method and apparatus for treating condensate in nuclear and thermal power stations. It is necessary that process for treatment of such condensate effectively remove all dissolved impurities, because the treated water must meet very strict purity requirements. In many instances the sodium and chloride concentrations in the treated condensate cannot be greater than 1.0 and 1.5 .mu.g/l, respectively.
According to known technology, the treatment of such condensates is normally carried out in vessels containing an intimate mixture of cationic and anionic ion exchange resins, i.e., in a mixed resin bed, the purpose of which is twofold. Thus, corrosion and erosion products are formed in the steam cycle. These are generally oxides of metals and erosions inside the circuit of the station that shuttles vapor are eliminated by filtering in the mixed resin bed. Dissolved compounds are introduced by leakage of condenser cooling water, and these dissolved impurities are eliminated by ion exchange in the mixed bed.
Experience has shown that only treatment by mixed beds can produce satisfactory effluent when the condensate has been subjected to condenser leaks.
Such known technology is at a disadvantage however since the steam and the condensates contain ammonia, with the pH of the condensate maintained between 9.2 and 9.6.
The presence of ammonia in the condensate leads to gradual exhaustion of the cationic resin of the mixed bed and reduces its ability to remove dissolved sodium ions. As is well known, there is an absorption ratio which is favorable to cationic resins with respect to NH.sub.4, that is preferentially absorbed through sodium. Thus, during processing of water containing ammonia and sodium ions, when ammonia absorption has reached a certain degree, sodium ions are freed by the resin, eluted by the ammonia. Therefore, the concentration of sodium ions in the effluent increases instead of diminishing.
Therefore, starting with the cationic resin being in balance with ammoniated condensate, NH.sub.4 -Na competition prevents removel of sodium ions, and it is no longer possible to ensure the effluent quality required by modern stations in the event of a leak, no matter how small, from the condensers. The concentration of sodium ions in the effluent exceeds acceptable limits, thereby requiring resin regeneration.
Such regeneration operations are lengthy and lead to significant consumption of the necessary regenerating reagents, e.g. sulfuric acid for regenerating cationic resins and sodium hydroxyde for regenerating anionic resins. One way to save the time required for the regeneration operations is to have a cationic resin bed precede the mixed bed. The cationic resin bed removes the ammonia from the condensate and prlongs the length of time of operation of the mixed resin bed. Thus, the cationic resin bed is regenerated each time it is saturated with ammonia, and the mixed bed exchanger remains operational until the anionic resin is exhausted.
In another known method, two beds of superimposed ion exchange resins are contained in the same vessel, the two beds being separated by a grid, and the two beds being separately regenerated. The separating mechanial grid is designed to maintain the lower bed in place during the extraction and replacement of the resin of the upper bed upon regeneration thereof. However, the use of the grid presents several disadvantages, the most significant of which is the need to provide a more complex overall installation which must be equipped not only with a grid, but also with components for evacuating the upper bed to allow for regeneration and rinsing. Additionally, the grid must be evenly supported.
Further, it does not appear that this prior method of separate beds has been used in the specific purification operation discussed herein.